Whilst traditionally, vehicles particularly heavy goods vehicles have been powered by diesel, there is an ever increasing demand for vehicles that can run completely, or partially on alternative fuels such as liquefied natural gas (LNG). As this demand grows, there is a need to be able to efficiently store an alternative fuel such as LNG.
It is known to store a cryogenic liquid such as LNG in a double skinned vacuum insulated vessel. This helps to extend the “shelf life” of the LNG and maintains the LNG in a liquid form. LNG in its liquid form is in its most condensed state compared to a compressed gas and therefore takes up a minimum amount of space.
It is known to mount such a tank in a space which exists in the vehicle well between two of the wheels of a vehicle. There are therefore restrictions on the size and shape of a storage tank suitable for storing a cryogenic liquid that may be mounted on vehicles such as a heavy goods vehicles, buses, vans, cars, etc.
Existing cryogenic storage tanks for non-vehicular use are substantially cylindrical in shape and are generally designed to be mounted vertically, i.e., with the axis of the cylinder extending substantially vertically.
It is known to adapt such vertical storage tanks for use as vehicle storage tanks. However, due to the space available in vehicles, it is necessary to mount the storage tank so that the axis of the tank extends substantially horizontally in use. This leads to problems. For example, valves and pipes extending from a storage tank designed to be mounted vertically, extend generally through one end of the tank. This is disadvantageous when the tank is mounted horizontally on a vehicle, because the valves and pipes take up valuable space which could otherwise be used as part of the storage volume. In addition, due to the limited space available in the vehicle wheel well, access to the valves extending from the tank is also extremely limited.
Because it is necessary to allow sufficient space for the pipes and valves in order to allow access to these pipes and valves, the storage capacity of the tank must be reduced relative to the space available, in order that the tank assembly, including the pipes and vessels, can fit into the space available on the vehicle wheel well.
A further problem associated with existing storage tanks for storing cryogenic liquids is that cold tracking is possible through the neck of the vessel which leads to ice formation. This occurs because the inner vessel is generally fixed to the outer vessel at one end thus allowing the opposite end of the inner vessel to expand or contract relative to the outer vessel, as appropriate depending on the state of the cryogenic liquid within the tank. Cold tracking may therefore take place across points of contact between the inner vessel and the outer vessel.
Cold tracking is a term used to describe the effect of a thermal gradient between a cold surface and a warmer surface. This means that in the case of double skinned cryogenic storage tanks, at any point where there is contact between a cold surface and a warmer surface, the temperature of the warmer surface will be reduced due to cold tracking, which in turn results in the temperature within the tank increasing. This is disadvantageous because the temperature of the liquid stored in the cryogenic tank will rise as a result of the cold tracking, causing the liquid to vaporise. This leads to the pressure within the cryogenic tank increasing.
Another problem with existing storage tanks is that because tanks are generally designed to be mounted vertically but then are mounted horizontally for vehicular use, manifolds connecting pipework into and out of the storage tank may be submerged by cryogenic liquid during use of the tank. This is undesirable, as it increases cold tracking.
Another problem with existing storage tanks for storing cryogenics liquids is that is can be possible, under certain circumstances, to overfill the storage tank with the cryogenic liquid. This can lead to a build up of pressure within the tank, if, for example the liquid expands due to solar heat gain.
In order to prevent overfilling, it is known to use an ullage management system comprising one or more ullage chambers or tubular vessels which initially prevent liquid flowing into the storage tank from entering the interior of the ullage chamber or tubular vessel. This means that once the remainder of the tank, not including the volume defined by the ullage management system, is filled with the cryogenic liquid, a detector may indicate to a user that the tank is full. The user may then stop filling the tank with further cryogenic liquid.
The ullage management system allows the liquid to slowly enter the ullage chamber or vessel after the tank has been filled thus creating a volume in the inner tank that is empty, and that is therefore available for expansion should the temperature conditions within the tank result in the liquid expanding. This helps to prevent the pressure in the tank from increasing.